Integrated circuits are formed on a semiconductor substrate, which is typically composed of silicon. Such formation of integrated circuits involves sequentially forming or depositing multiple electrically conductive and insulative layers in or on the substrate. Etching processes may then be used to form geometric patterns in the layers or vias for electrical contact between the layers. Etching processes include “wet” etching, in which one or more chemical reagents are brought into direct contact with the substrate, and “dry” etching, such as plasma etching.
Various types of plasma etching processes are known in the art, including plasma etching, reactive ion (RI) etching and reactive ion beam etching. In each of these plasma processes, a gas is first introduced into a reaction chamber and then plasma is generated from the gas. This is accomplished by dissociation of the gas into ions, free radicals and electrons by using an RF (radio frequency) generator, which includes one or more electrodes. The electrodes are accelerated in an electric field generated by the electrodes, and the energized electrons strike gas molecules to form additional ions, free radicals and electrons, which strike additional gas molecules, and the plasma eventually becomes self-sustaining. The ions, free radicals and electrons in the plasma react chemically with the layer material on the semiconductor wafer to form residual products which leave the wafer surface and thus, etch the material from the wafer.
As discussed above, plasma includes high-energy ions, free radicals and electrons which react chemically with the surface material of the semiconductor wafer to form reaction produces that leave the wafer surface, thereby etching a geometrical pattern or a via in a wafer layer. Plasma intensity depends on the type of etchant gas or gases used, as well as the etchant gas pressure and temperature and the radio frequency generated at an electrode in the process chamber by an RF generator. If any of these factors changes during the process, the plasma intensity may increase or decrease with respect to the plasma intensity level required for optimum etching in a particular application. Decreased plasma intensity results in decreased, and thus incomplete, etching. Increased plasma intensity, on the other hand, can cause over etching and plasma-induced damage of the wafers. Plasma-induced damage includes trapped interface charges, material defects migration into bulk materials, and contamination caused by the deposition of etch products on material surfaces. Etch damage induced by reactive plasma can alter the qualities of sensitive IC components such as Schottky diodes, the rectifying capability of which can be reduced considerably. Heavy-polymer deposition during oxide contact hole etching may cause high-contact resistance.
Furthermore, plasma-etching techniques are incapable of discriminating between the layer or layers to be etched and the underlying layer or layers, which should remain unaffected by the etching process. For these reasons, the plasma reactor must be equipped with a monitor that indicates when the etching process is to be stopped. Such a monitor may utilize an end-point system or mode to terminate etching in order to prevent undesired etching of the underlying layer on the wafer.
One type of end point detection system commonly used in plasma etching processes is optical emission spectroscopy, which analyzes the light emitted by energized atoms and molecules in the gas discharge leading from the etching chamber. This is accomplished by using a detector equipped with a filter which lets light of a specific wavelength penetrate to the detector to analyze the concentration of excited products or reactants during the etching process. The emission signal generated by the gas discharge begins to rise or fall at the end of the etch cycle, thus indicating that material of a different chemical composition (that of the underlying layer) than that of the etched layer is being etched from the wafer surface.
Another end-point detection system includes laser inferometry, in which laser beams are directed toward the etched wafer surface. If the films on the wafer surface are transparent, then the laser beams reflected from the top and bottom of the etched layer interfere with each other. As the etching process reduces the thickness of the etched layer, the degree of interference between the laser beams changes. The elapsed time between the light maxima and light minima can be used to determine the etching rate. At the end of the etching process, the interference between the beams stops and the interference signal flattens out.
In contact etching processes, contact openings, or vias, are etched in an insulative layer to provide electrical contact between a conductive layer which underlies the insulative layer and a second conductive layer to subsequently be deposited on the insulative layer. In contact etching processes, the end point mode of determining the suitable end of an etching process cannot be used due to the relatively low exposure rate of the insulative layer to the plasma and because the plasma encounters no obvious stop layer to indicate when the etching process should be stopped. Therefore, a time mode is typically used to determine the end of contact etching processes.
According to the time mode, the time for plasma generation is programmed into the etcher. When the etch time has elapsed, plasma generation in the etcher may be manually or automatically terminated or attenuated at this point to prevent over etching of the semiconductor. However, the time mode fails to provide any indication of abnormal chamber conditions in the event that the plasma-forming source gas fails to initially ignite and generate the plasma in the chamber or the plasma intensity rises too high or falls too low for optimum etching. Consequently, batches of wafers may be under- or over-etched and require discarding.
Referring to the schematic of FIG. 1, a conventional plasma etching system, such as an MxP+ chamber available from the Applied Materials Corp. of Santa Clara, Calif., is generally indicated by reference numeral 10. The etching system 10 includes a reaction chamber 12 having a typically grounded chamber wall 14. A cathode 16 is positioned in the bottom portion of the chamber 12, and an electrostatic chuck 18 is provided on the cathode 16 for supporting a wafer 20 thereon. Plasma-generating source gases are introduced into the reaction chamber 12 through multiple openings 23 of a GDP (gas distribution plate) or showerhead 22 provided in the top of the reaction chamber 12. Volatile reaction products and unreacted plasma species are removed from the reaction chamber 12 by a gas removal system (not shown).
The etching system 10 further includes an end point detector system 26 which utilizes optical interferometry to detect the endpoint of the etching process. The end point detector system 26 includes a port 28 which is mounted in the side wall 14 of the reaction chamber 12 and includes a quartz window 29 recessed in a port opening 30 (FIG. 2). A fiber optic cable 34 connects the port 28 to a controller 36. Accordingly, during operation of the etching system 10, UV or visible light rays 33 are reflected from the wafer 20 and penetrate the quartz window 29 to the light sensor 32. The light sensor 32 is capable of measuring the constructive and destructive interference between the UV or visible light rays reflected off the etched layer on the wafer 20 as the layer on the wafer 20 changes from one material interface to another, in conventional fashion. The light sensor 32 thus continually senses the thickness of the layers during etching, and this data is sent to the controller 36. When the desired thickness of the layer or layers on the wafer 20 has been reached, the controller 36 terminates the etching process.
As shown in FIG. 2, over a prolonged period of continuous usage of the etching system 10, a layer of polymer deposition 38 typically accumulates on the port 28, including the quartz window 29 thereof, due to contact of plasma with these surfaces. This tends to interfere with accurate monitoring of the layers being etched on the wafer 20, since light reflected from the wafer 20 is inaccurately and incompletely transmitted through the quartz window 29 to the light sensor 32. Consequently, the port 28 and quartz window 29 must be cleaned and the polymer deposition 38 removed therefrom before use of the etching system 10 can be continued. Typically, about 2000 wafers 20 can be etched between periodic maintenance cleanings of the reaction chamber 12. However, periodic maintenance cleanings require inactivation of the reaction chamber 12 and this reduces throughput of wafers 20 in the etching system 10. Accordingly, a quartz window which is capable of prolonging the time required between periodic chamber cleanings is needed for increasing wafer throughput in an etching system.